You can take them to Nevada or any other State where they are legal and sell them, but the transfer will have to go through an FFL. So you and the buyer would meet at an FFL in the other State and do the transfer. The FFL is going to charge a fee for this service and some FFLs don't like to do any transfers that involve a Californian.

Another option if you find a buyer from another State is to have a California licensed dealer who also has an assault weapons permit ship the gun to the out of state buyer's local FFL. In this case both FFLs will charge something.

According to a person I talked to at the CA DOJ, it is legal for a private party to ship their personally owned registered AW to a location out of state. Now, this does not seem to match with what the law says, so you should get it in writing before doing so.